It is our first winter in a new-to-us house, that has a pretty flat roof (3/12-to-4/12 pitch), and a ridge vent along the entire top of the roof. We just had the snowiest December on record (Minnesota), which put a good 36" of snow on the roof, burying the ridge vent. I have been up on the roof, and have cleared most of the snow, but we have a good 4' ice-dam that has bridged to/filled the gutters (and is a good 4" over the top of the gutters currently). My primary concern, is that water seems to be getting behind the facia, into the eaves, then coming out on the outside of the house, and running down the exterior siding where it freezes. So far there is no apparent water damage inside, but I want to get this cleared up ASAP before it gets into the house.
My questions are-
1) What is the best way to get rid of the ice dam (the entire edge of the roof, on both sides of the house, 55' on each side, has an ice dam going 4-6' up the roof)? I have thrown a bunch of potassium chloride up on the roof, but it didn't seem to do too much. I'll try the nylon tube mentioned in the answer here, but how should I orient it to get the most ice off the roof?
2) How do I prevent this in the future? I have a roof rake, and can rake the first 6-8' of the roof, but does that do any good if the ridge vent is blocked? It sounds like the heat tape doesn't work all that well. Thoughts?
Edit- Well, the nylon stockings with potassium chloride have been on the roof for 8 hours now, and they have yet to melt anything... hummm...